BGE: bge.texture API documentation enhancement

This patch attempts to improve and review the documentation of bge.texture, as requested in the [[ http://wiki.blender.org/index.php/Dev:2.5/Source/Development/Todo/GameEngine#Video_Texture | TODO list ]].

More specifically, it

- fixes the rst syntax, including titles of the examples bge.texture.py and bge.texture.1.py;

- adds, standardizes and reviews description of the API elements, particularly signatures, types, etc.

- adds SOURCE_* constants to the doc

- splits the doc into thematical parts (Video, Image, Texture, and Filter Classes, Functions, Constants).

Notes:

- The parameter "mode" of ImageBuff.plot has to be described better. Actually, the whole set of IMB_BLEND_* constants (from IMB_imbuf.h) should be exposed to Python. I'll do that in a future diff, and complete the doc at the same moment (adding those IMB_BLEND_* constants to the Constants part of this doc).

- The option of using webcams in VideoFFmpeg is still particularly not well documented. I am planning to make a proposal about fixing T18634 (and its corresponding TODO in the list) by integrating OpenCV in the BGE (and Blender?). The idea would then probably be to add a new class, f.ex. ImageWebcam, making this functionnality more specialized. So for now I don't think it is worth to document that part much.

This patch fixes T44284 too.

Reviewers: moguri, kupoman, campbellbarton, panzergame, lordloki

Reviewed By: panzergame, lordloki

Subscribers: hg1

Projects: #game_engine, #game_python, #documentation

Maniphest Tasks: T44284

Differential Revision: https://developer.blender.org/D1352